1. Field of the Invention
This invention relates to the production of synthetic fuels and is particularly concerned with a process wherein a synthesis gas is methanated to produce a substitute pipeline gas having a heating value of about 900 to 1000 BTU per standard cubic foot from feed gas comprising carbon monoxide and hydrogen.
2. Description of the Prior Art
In the fixed bed catalytic methanation of gases containing carbon monoxide and hydrogen, the reaction between the carbon monoxide and hydrogen is very exothermic and, if not controlled within the reactor, can cause sintering of the catalyst, carbon deposition on the catalyst and/or thermal cracking of the product methane to carbon and hydrogen. Carbon formation through thermal cracking and/or carbon monoxide disproportionation in turn has a tendency to foul the catalyst bed. Furthermore, most nickel catalysts active for the methanation reactions will tend to deactivate at high temperatures. Consequently, it is of importance to limit the maximum temperature of the catalyst bed. Also, it is important that the gas enter at the lowest inlet temperature which will still give an acceptable initiation reaction rate and still prevent the formation of a carbonyl compound which can occur through the reaction of the carbon monoxide with the catalyst at temperatures below proper operating temperatures.
To overcome some of these problems caused by overheating or carbonyl formation, extensive recycle streams are used as diluent to absorb some of the exothermic heat evolved. Additional measures for avoiding too high temperatures in the reactor include cooling of the catalyst bed or of the reaction gases. For example, direct cold gas recycle and internal cooling of the reactor by heat transfer surfaces within the bed are recognized methods by which temperature controls may be effected. Local heating is difficult to avoid when using the latter and the building of internal exchange surfaces tends to be expensive. The hot gas recycle and direct cold gas recycle methods, on the other hand, require high recycle ratios. As a consequence, large pressure drops through the catalyst beds occur and the requirements for compressor power and strictor design specifications increase proportionately, hence increasing compression construction costs.
Catalytic methanation processes which employ a separate water gas shift conversion step and avoid using either a high recycle gas rate and/or very large heat removal between stages include U.S. Pat. Nos. 3,890,113 and 3,904,389.